The present invention involves microwave cooking. More particularly, the present invention relates to a printed microwave susceptor structure or package for safely cooking and browning foods in a microwave oven.
The cooking of foods in a microwave oven differs significantly from the cooking of foods in a conventional oven. In a conventional oven, heat energy is applied to the exterior of the surface of food, which moves inwardly until the food is cooked. Thus, food cooked conventionally is typically hot on the outer surfaces and warm in the center. Meanwhile, microwave cooking involves the absorption of microwave energy which characteristically penetrates far deeper into the food than does the heat energy in conventional cooking. Also, in microwave cooking, the air temperature in the microwave oven may be relatively low. Therefore, it is not uncommon for food cooked in a microwave oven to be cool on the outer surfaces and much hotter in the center.
Thus, in order to make the exterior surfaces of microwave cooked food brown and crisp, the exterior surfaces of the food must be heated to a sufficient degree such that moisture on the exterior surfaces of the food is driven away. Since the exterior surfaces of food cooked in a microwave oven are typically cooler than the interior of the food, it is difficult to brown food and make it crisp in a microwave oven. In order to facilitate the browning and crisping of food cooked in a microwave oven, devices known as susceptors have been developed. Susceptors are devices which, when exposed to microwave energy, become very hot. By placing a susceptor next to a food product in a microwave oven, the surface of the food product in contact with the susceptor is heated and becomes crisp.
A typical susceptor structure comprises a substrate such as paper or paperboard in combination with a microwave interactive material which absorbs microwave energy. For example, susceptor structures may be prepared using a thin layer of metal such as aluminum applied to a piece of film which is laminated to the substrate. Susceptors of this type are generally referred to as metallized structures. Other forms of susceptors may use coating, spraying or printing processes wherein a material capable of absorbing microwave energy is applied to the substrate. These susceptors are generally characterized as non-metallized structures. In the prior art constructions, the microwave interactive material is designed to be in direct contact with the food product, or as close as possible to the food product, separable therefrom only by a thin layer of paper, film, or the like. In fact, while there are literally hundreds of prior art United States patents granted for food packaging including the use of microwave susceptors, only a handful of these constructions have actually reached commercial use. The problems inherent in most prior art structures of the non-metallized type involve the development of hot spots, or uneven and runaway heating upon exposure to microwave energy, which causes charring and degradation of the paper or paperboard substrates during use, and the fear of potential migration of contaminants from the microwave interactive materials of the susceptor layer into the food products being cooked.
A number of attempts have been made in the past to overcome the development of hot spots and runaway heating in non-metallized susceptors including, the use of heat attenuators in the susceptor coating itself, or applied as an independent layer to the substrate (U.S. Pat. No. 5,285,040); the varying of the coverage of printed or coated microwave interactive materials between the regions of the packaging in contact with the food products, and the regions of the packaging adjacent to the food products (U.S. Pat. No. 4,970,358); and with the use of thermal barrier layers between the susceptor layer and the substrate (U.S. Pat. No. 5,231,268). The introduction of a thermal barrier layer between the susceptor layer and the substrate, as disclosed in the '268 patent, has for the most part solved the problem of charring and degradation of the substrate layer during use, but a practical solution to the problem of unwanted migration of contaminants from the microwave interactive materials in the susceptor layer has yet to be resolved.
At least some protection from migration of contaminants can be achieved by simply placing the microwave interactive material layer on the opposite side of the substrate from the food contact surface (U.S. Pat. Nos. 4,190,757 and 5,153,402). In like manner, the susceptor layer containing the microwave interactive material can be sandwiched between two substrates of different thickness (U.S. Pat. No. 5,012,068), or insulated from the substrate by multiple coatings (U.S. Pat. No. 5,006,405), to achieve some protection from migration of contaminants. However, there is a continuing need for the development of printed or coated microwave susceptor structures which are capable of controlled heating and which are safe for use.